1. Field of the Invention
The present invention relates to a fiber optical displacement sensor to be used in a liquid level sensor, a pressure gauge or a thermometer.
2. Description of the Background Art
A fiber optical displacement sensor has been utilized in various fields. A most common application includes that in determining a distance to the target, such as in a liquid level sensor for detecting a position of a liquid surface to determine an amount of liquid in a container.
In a typical such liquid level sensor using a fiber optical displacement sensor, a light beam from a light source is projected, through a projecting optical fiber between the light source and a sensor head, onto a surface of the liquid contained in a container located below the sensor head. A reflected beam is received, through a receiving optical fiber between the sensor head and a first photo diode, by the first photo, diode. Such a liquid level sensor is also equipped with a reference optical fiber which propagates the light beam from the light source to a second photo diode, along the projecting optical fiber and the receiving optical fiber, through the sensor head, but without projecting the light beam on the liquid surface. A reflection signal from the first photo diode and a reference signal from the second photo diode are arithmetically operated by an operational circuit to adjust effects such as light intensity fluctuation due to external disturbances, and an accurate measurement value is obtained by an output adjusting circuit which derives a correspondence between a signal from the operational circuit and a liquid level.
Such a conventional liquid level sensor using a fiber optical displacement sensor is known to have the following problems.
First, it has been known that a conventional fiber optical displacement sensor has a rather limited range of measurable distances or displacements so that its use has been limited to applications in which only short distances are dealt with.
Second, such a conventional liquid level sensor usually incorporates optical fiber multi-connectors at junction between the optical fibers and the sensor head or at a middle of optical fibers for the practical purpose of being able to disassemble the entire instrument. In such a case, it is necessary for the optical fiber multi-connectors to have less than 1 .mu.m precision in their positioning in order to be able to reproduce satisfactory light intensity, but incorporation of such a high precision optical fiber multi-connectors is both impractical from a standpoint of cost and troublesome from a standpoint of ease in handling.
Thirdly, the use of multiple optical fibers such as the projecting optical fiber with the reference optical fiber or the receiving optical fiber with the reference optical fiber makes it easier to receive more influences from the external disturbances.
Also, since the intensity of the reflection beams from the liquid surface is very low, in order to improve the precision of the measurement, it is necessary not only to improve couplings between the receiving optical fiber and the first photo diode as well as between the reference optical fiber and the second photo diode but also to selects the first photo diode and the second photo diode of the equal characteristics. However, there is a limit in an equality of the characteristics between two photo diodes, so that the use of two photo diodes restricted the precision of the measurement.
There are also other applications of a fiber optical displacement sensor such as that in a pressure gauge using a diaphragm in which the displacement of the diaphragm is detected to determine a pressure exerted on the diaphragm. Such a pressure gauge using a fiber optical displacement sensor has an advantage of being free of an influences due to electric noises caused by things such as an electromagnetic induction. But the similar problems given above for the liquid level sensor are also present in such applications.